Groundwater Recharge in a Coldwater Stream Watershed during Urbanization

Urban development of rural land is of concern for water resources. The quantity and quality of surface runoff and groundwater recharge can be significantly affected by urbanization. Base flow in streams and cold water habitat, e.g. for trout, depend on groundwater. If water recharge to aquifers is reduced, and surface runoff is increased, cold-water fish habitat can be adversely affected. The change to groundwater recharge resulting from the urbanization of a rural/natural area in the Vermillion River watershed in Minnesota was investigated. The Vermillion River is a groundwater-fed designated trout stream at the southern fringes of the Minneapolis/St. Paul metropolitan area in Minnesota. In this watershed urban development has encroached on farmland and natural areas in the last 25 years. The process is projected to continue into the future. Three studies related to groundwater recharge were conducted: (1) a soil water budget study to estimate the influence of changed imperviousness and surface vegetation on natural recharge in a small tributary watershed of the Vermillion River, (2) a trend analysis of stream/base flow at the USGS stream gauging site on the Vermillion River near Empire, MN, and (3) a water use study to estimate the influence of imported water on artificial recharge stemming from. The results of the first study confirm that the increase in impervious surface area associated with urban development will decrease annual natural groundwater recharge. The trend analysis (second study) showed no statistically significant trend in the streamflow record during the period of 1982 to 2006 even though imperviousness in the watershed increased from 8% in 1984 to 13% in 2005. The third study revealed that groundwater recharge from urban water supply and drainage systems and from irrigation has more than doubled from 1982 to 2006; it accounts for nearly 10% of annual recharge in the watershed and matches the reduction in natural recharge predicted by the soil water budget models (first study). The net effect of urbanization on groundwater recharge, seen in the trend analysis of the Vermillion River base flow was close to zero.